What under-inflation does

A tyre 0.4 bar below placard looks almost normal at a glance. The sidewall bulges a little more. The contact patch grows at the shoulders. Flex in those shoulders is heat.

Fuel use rises. Steering feels vague. On a wet M25 the outer ribs can aquaplane while the centre still looks fine. Slow leaks through a corroded rim or a dry valve core are enough.

TPMS is required to warn at about 25% below the placard. On a 2.4 bar car that is already a soft tyre. Treat the lamp as an emergency, not a reminder for “sometime this month.”

Chronic under-inflation leaves both shoulders gone and the centre proud. That tyre may still have legal groove in the middle and be scrap at the edges.

If one tyre is low and the others match, find the hole. Filling the same corner every fortnight is a repair job.